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    SubjectRe: is irq smp affinity good for anything?
    On Tue, 11 Mar 2003 wrote:

    > if IRQ affinity cannot help in interrupt latency and
    > interrupt auto/balancing get's the same throughtput
    > as irq binding... what is irq smp affinity for???
    > from this mail from intel
    > I understand that intel's interrupt auto/balancing works equal that manually
    > tunning the interrupts....
    > so?
    > Ulisses
    > On Mon, Mar 10, 2003 at 11:22:22PM +0100, wrote:
    > >
    > > Hi all
    > >
    > > I have measured interrupt latency of a bi-processor system with akpm's intlat/timepeg
    > > utilities and kernel 2.4.20. The system uses is a two-way PIII@800Mhz
    > > -- Intel motherboard, ISP 2100 if I remember ok, with a SCSI disk on a aic-7896
    > >
    > > I tried to know if I could reduce the latency of an interrupt handler by
    > > binding this handler to a particular cpu and not allowing any other
    > > interrupt to execute in that cpu.
    > >
    > > I found that overall latency increases, but also the latency on both cpus,
    > > that is, I could not reduce the latency on the interrupt I was interested
    > >
    > > I also tried to disallowing just scsi & ethernet handlers to execute on the
    > > cpu in wich I'm binding the interrupt handler I'm insterested with, I get
    > > similar results
    > >
    > > the interrupt handler I'm interested with is an ATM card receiving
    > > around 6000 interrupts/second
    > >

    33 MHz machines easily handle 6,000 interrupts per second --
    unless you are trying to execute code within that interrupt
    that requires 1/6000th of a second to execute! Perhaps it's
    not a "latency" problem, but an interrupt code-bloat problem
    where most of the stuff should be executed out of the interrupt

    There are timer queues and kernel threads available that might
    help you. Also a tightly-coupled user-mode daemon that uses
    your driver only to interface with the hardware and not do
    any "logic" is probably the way to go.

    Dick Johnson
    Penguin : Linux version 2.4.20 on an i686 machine (797.90 BogoMips).
    Why is the government concerned about the lunatic fringe? Think about it.

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